1. Field
The present teachings relate to switches, and particularly to a switch circuit for switching radio frequency (RF) signals. The RF switch, with a novel bleeder circuit, may be used in RF applications wherein a specified switch performance is required when normal operating power is turned off. In one embodiment, the RF switch comprises enhancement and depletion mode devices implemented as an integrated circuit on a silicon-on-insulator (SOI) substrate.
2. Description of Related Art
Radio frequency (RF) switches for directing RF signals are found in many different RF devices such as televisions, video recorders, cable television equipment, cellular telephones, wireless pagers, wireless infrastructure equipment, and satellite communications equipment. As is well known, the performance of RF switches is controlled by three primary operating performance parameters: insertion loss, switch isolation, and the “1 dB compression point.” These three performance parameters are tightly coupled, and any one parameter can be emphasized in the design of RF switch components at the expense of others. A fourth performance parameter that is occasionally considered in the design of RF switches is commonly referred to as the switching time or switching speed (defined as the time required to turn one side of a switch on and turn the other side off). Other characteristics that are important in RF switch design include ease and degree (or level) of integration of the RF switch, complexity, yield, return loss, and cost of manufacture.
For RF devices there are many Federal Communication Commission (FCC) standards that must be complied with in design and operation. One well-known standard, designated as FCC Part 15.115, specifies minimum isolation requirements between different input nodes and output nodes for RF switches. For example, the RF switch in a television that switches the RF tuner input between an antenna and a cable TV signal must comply with this standard. Furthermore, these isolation requirements must be maintained even when the RF devices are unplugged or turned off.
Other RF switches may be required to assume an appropriate default operating state and performance when the RF switch is not provided with operating power (i.e., when the switch is “unpowered”). For example, a switch in a video recorder that switches the recorder output between an internal modulator signal and an external RF signal (e.g., cable TV or antenna) is required to default to a low-loss connection for the external RF signal when the video recorder is unplugged or turned off.
An additional requirement for many RF switches involves maintaining proper termination impedance for the RF signal. For example, the Data Over Cable Service Internet Specification (DOCSIS) requires that an appropriate termination impedance (e.g., 75 ohms) for a cable TV signal be maintained by the input of an RF switch even when the RF switch is unpowered.
A challenge associated with designing RF switches capable of unpowered operation using integrated circuits arises from the fact that a transistor generally requires non-zero bias levels for proper operation. For example, if the gate of a field-effect transistor (FET) is not provided with a controlled DC bias level, the FET may assume an on-state, an off-state, or somewhere in between, depending on the type of FET, RF signal, and other circuit elements coupled to the gate. Further, unpowered solid-state switches include non-linear elements. These can cause rectification of RF signals and undesired bias conditions. According to prior art, relays have been used for RF switches that must meet performance specifications for unpowered operation. Although relays can be designed to assume desired default operation when unpowered, relays are bulky, expensive, unreliable, require high current to operate, and have limited switching speeds. Consequently, a need exists for an RF switch that meets performance requirements when unpowered, and that can be readily fabricated as a monolithic integrated circuit.